Hi Peter,
I have posted before on this subject, saying that I've been so busy working on other projects and customer bikes that I have not yet got mine finished
To answer you questions ( I hope??)
From your message it sounds like the dommie clutch was bored to fit on the BSA shaft?? is this the case?
From which era is your GS550 clutch from?
The modern GS550 clutches have a splined centre 16 x 20 mm, If you can find a gearbox shaft from a plunger A7/10 this will fit the swing arm gearbox and has a spline slightly bigger than the GS,
This is where I'm held up with mine as I want to find time to sort out a slotting attachment to modify the splines
The other method of fitting a Suzuki clutch is to fit one to the 4 spring Triumph shaft adaptor, as on the Pearson Clutch
This requires a bearing 1 3/8 x 2 1/2 x 3/8in these are fairly expensive
The modern GS 550 clutch does not (in my opinion) have enough metal in the centre to open out to fit the shaft adaptor !
I examined a Pearson clutch (at an autojumble) and saw that there was wear on the alloy spline where it fits the shaft adaptor
My solution for this is to get a centre from a GSXR 750 (1989/90?) this has a steel piece cast into the centre which looks big enough to bore and spline to the Triumph type adaptor
It will also need a little turned off the outside diamater to fit into the GS basket, or use the 750 basket
I havnt checked if the 750 outer will fit in the BSAprimary case (yet)
HTH
John O R